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Colloids May Enable Integrated Photonic Circuits

Photonics Spectra
Jun 2002
Hank Hogan

Thanks to researchers at the University of Toronto, photonic applications one day could run on groovy crystals. The crystals -- composed of colloidal spheres deposited on a grooved substrate -- could enable the creation of integrated photonic circuits.

Professors Eugenia Kumacheva and Edward H. Sargent and their students have developed a method of producing regular arrays of colloid crystals. In principle, the technique may be used with any conducting surface, such as doped silicon, gallium arsenide or indium phosphide.

Colloidfine5blue.jpg
Researchers at the University of Toronto have developed a technique that enables the construction of colloidal photonic crystals. The application of an electric field forces the colloidal particles into grooves that have been patterned on the substrate, seen here in a scanning electron microscope image. Courtesy of Eugenia Kumacheva.

The technique developed out of the researchers' experiments with 580-nm-diameter spheres made of poly(methylmethacrylate). They began by depositing a photoresist on a conducting substrate, which they patterned into a series of flat hills and deep valleys. The grooved surface formed the top electrode of the device, while the bottom was a flat plate. In the next step, they dispersed the latex spheres into the 5-mm-wide gap between the plates. With the proper voltage, the spheres traveled upward and into the confined stripes where the conductor was exposed.

The scientists varied the width of the conducting region. Where it was wide, the sphere arrangement was fairly random. But when the spacing reached a critical width of approximately 4.7 µm, the arrangement became ordered as the spheres were forced into the area by the electric field. The nature of the packing depended on the ratio of the confinement width to the sphere diameter. They found that they could produce square and hexagonal packing by varying the distance between photoresist ridges.

This method can be combined with semiconductor and optoelectronics materials processing to create precise, ordered photonic crystals. The result would be a photonic-lattice device with desired optical and electrical characteristics.

The researchers are working to optimize the process and create specific structures.


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